Lasers that are tunable over a wide range of wavelengths and have a narrow linewidth are desirable for a number of applications, including, for example, laser radar, isotope separation, remote sensing, medicine, and lithography. One such tunable laser uses a diffraction grating inside the laser cavity and a gain medium that, when pumped, fluoresces over a broad range of wavelengths. The angle of the diffraction grating inside this so-called Littman cavity is adjusted so that only the desired wavelength of light is amplified by the cavity.
The main difficulty with the Littman cavity is that its grating has a low diffraction efficiency. Most of the cavity light is lost due to ordinary mirror, or specular, reflection from the grating. Therefore the Littman cavity gives only a low powered laser.
This problem was partially solved by Lee, Cha, Kim, and Ko in Optics Letters 20 (1995) pp. 710-712 and U.S. Pat. No. 5,633,884. Lee et al. use a Littman cavity coupled to a second, slave oscillator. The light in the Littman cavity is amplified by the slave oscillator to overcome the power restrictions of the Littman cavity operating alone.
In this coupled oscillator approach, the gain medium is activated using an external laser pulse. Light first builds up in the Littman cavity. The power in the Littman cavity then "seeds" the gain medium, whose induced emission circulates in and is amplified by the slave oscillator. The light circulating in the slave oscillator is specularly reflected from the grating. Therefore, light that was previously lost to specular reflection in the Littman cavity alone is recaptured and amplified in the method of Lee et al. The self-seeding coupled oscillators yield a laser with increased power.
However, the laser of Lee et al. still has some drawbacks. For the narrowest linewidth possible, the laser should operate in a single longitudinal mode. However, the laser of Lee et al. typically operates in several longitudinal modes at once, thereby giving a broader than optimum linewidth.